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An adaptive immune response implies expansion of activated T cells and subsequent elimination to maintain homeostasis in a process called activation-induced cell death. Some cells, however, differentiate into memory cells and ensure a strong secondary immune response. To analyze the apoptosis phenotype of memory T cells on a cellular and molecular level, we have established an in vitro model of T cell activation and generation of cells phenotypically and functionally similar to memory cells. These long-term cultured T cells show a CD95-resistant phenotype, although they are still sensitive towards TCR/CD3-mediated apoptosis. Biochemical analysis revealed that these cells shift from CD95 type I (direct signaling from the receptor) during the effector phase to CD95 type II cells (dependent on the mitochondrial amplification loop). Moreover, their mitochondria are protected, probably due to high expression levels of Bcl-x(L) and Bcl-2. Thus, our data suggest a mechanism how memory T cells acquire resistance towards bystander cell death via the CD95 system.

Glucocorticoids (GCs) play an important role in the regulation of peripheral T-cell survival. Their molecular mechanism of action and the question of whether they have the ability to inhibit apoptosis in vivo, however, are not fully elucidated. Signal transduction through the glucocorticoid receptor (GR) is complex and involves different pathways. Therefore, we used mice with T-cell-specific inactivation of the GR as well as mice with a function-selective mutation in the GR to determine the signaling mechanism. Evidence is presented for a functional role of direct binding of the GR to 2 negative glucocorticoid regulatory elements (nGREs) in the CD95 (APO-1/Fas) ligand (L) promoter. Binding of GRs to these nGREs reduces activation-induced CD95L expression in T cells. These in vitro results are fully supported by data obtained in vivo. Administration of GCs to mice leads to inhibition of activation-induced cell death (AICD). Thus, GC-mediated inhibition of CD95L expression of activated T cells might contribute to the anti-inflammatory function of steroid drugs.

Apoptosis of activated peripheral T cells during the termination phase of an immune response is critical to maintain T-cell homeostasis. Activated T cells can be removed by two mechanisms: activation-induced cell death (AICD) and death by neglect. AICD is triggered by death receptors, whereas death by neglect is induced by cytokine withdrawal. CD95 (APO-1/Fas) belongs to the subfamily of death receptors and plays a major role in AICD. In this review, we focus on the molecular mechanisms of AICD, in particular those involving the CD95 system. Moreover, we discuss the relative contribution of AICD and death by neglect to terminate a T-cell immune response. In order to become fully activated, T cells require a second signal provided by antigen-presenting cells. We discuss how these costimulatory signals counteract pro-apoptotic signals and, finally, which signals might protect T cells from death to generate a pool of memory T cells.